Brennan's management company, Unified Management, told TheWrap that she passed away in a Burbank hospice after a battle with bladder cancer.
Brennan was born in 1932 in Los Angeles to parents Regina Manahan, a silent film actress, and John Gerald Brennan, a doctor. After studying at Georgetown University in Washington, D.C. and then the American Academy of Dramatic Arts in New York, Brennan landed the title role in the off-Broadway production "Little Mary Sunshine" in 1959. Her performance earned her an Obie Award and a "Promising New Personality" Theatre World Award alongside Warren Beatty, Jane Fonda and Carol Burnett.
After two years of playing Irene Malloy alongside Carol Channing's Dolly Levi Gallagher in the original 1964 Broadway production of "Hello, Dolly," Brennan came to Hollywood to join Dustin Hoffman in the TV movie, "The Star Wagon," followed by a supporting role in 1967 comedy "Divorce American Style," which starred Debbie Reynolds and Dick Van Dyke.
In 1971, Brennan's career took a turn for the dramatic when she starred as worn-out Texas waitress Genevieve in Peter Bogdanovich's "The Last Picture Show." Other memorable titles in her long filmography stretching through five decades include 1974 Best Picture Oscar winner, "The Sting," 1976 crime comedy "Murder by Death," and 1985 caper comedy, "Clue," in which she starred as Mrs. Peacock.
After her performance in "Private Benjamin" earned her an Oscar nomination in 1981, Brennan continued to portray her Army captain character for three seasons on CBS' television adaptation of the comedy. The role earned Brennan both an Emmy and a Golden Globe.
Later in life, Brennan was a recuring guest star on WB family drama, "7th Heaven," as well as the NBC sitcom "Will & Grace."
She is survived by sons Sam and Patrick -- an actor who appeared in "Twilight Saga: Breaking Dawn -- Part 2" -- daughter-in-law Jessica, sister Kate and grandchildren Liam and Maggie.
Good actress and she had a good long life. Thoughts and prayers to family and friends, I know she will be missed.